The mechanisms of a number of flavoprotein enzymes are being studied. The question of whether a transient carbanion of the substrate is formed is being investigated with the enzymes D-amino acid oxidase and L-lactate oxidase. The mechanism of oxygen activation in hydroxylation reactions is being investigated with the flavoproteins p- hydroxy-benzoate hydroxylase, melilotate hydroxylase and phenol hydroxylase. These studies involve extensive use of rapid reaction techniques to monitor or trap intermediates. The complex metalloflavoprotein xanthine oxidase is also being investigated. Among the facets of this enzyme of particular interest to us is the determination of the mechanism of inhibition of the catalytic reaction by pteridine 6-aldehyde, a very potent inhibitor active in the range of 10 to the minus 9th power M.